Conventional oil production and/or drilling platforms fall into two broad categories, the floating platform and fixed or bottom founded platform. There are three sub-categories of fixed platforms, the pile or jacket supported type, the jack-up type, and the gravity base type.
The pile or jacket supported platform is the most common type of fixed platform. It typically comprises a deck supported above the water by a framework of columns affixed to the sea bed and coupled together by braces. The jack-up type of platform includes a buoyant deck having three or more jackable legs coupled thereto, which is floatable with its legs in a raised vertical position to a particular site. On site, the legs are lowered to the sea bed, the deck being raised above the water level by the legs. The gravity base platform generally comprises a relatively large unit, usually made of concrete, which may be floated vertically to a site and sunk to the ocean floor by ballasting. Fixed platforms are typically secured to the ocean floor by means of piles. The cross-sectional area of the base of the gravity base platform provides resistance to loading, whereas the piles of the jacket supported platform provide support for loads.
Floating platforms (also known as column stabilized platforms) come in several designs, but typically a floating platform comprises a deck supported by several large diameter columns, some of which may be interconnected by pontoons. A floating platform is generally anchored at a particular location by means of cables or chains under tension attached to anchors or permanent seafloor fixtures, or other tethering means.
Conventional platforms are not well adapted for oil production operations at offshore locations subject to conditions which may require the platform to be both relatively rigid and stable on site at times and to be safely moved off site at other times under various conditions, including adverse weather conditions. Such locations include the following: areas subject to severe weather conditions and periodic intrusion of heavy ice such as icebergs or ice floes; offshore areas controlled by politically unstable countries; and areas containing oil or gas fields of marginal commercial value.
The conventional floating platform is of course readily capable of movement off site. However, it is not well adapted for on site operations such as long term production of oil or gas, since these operations require a facility which is firmly affixed to the sea bed. Floating platforms tend to move about too much in heavy seas and during bad weather, resulting in stoppages of oil or gas production operations. Since the interruption of operations has a much more serious economic consequence in the case of oil or gas production than it does in the case of drilling, floating platforms of a type that are suitable for most drilling projects have serious limitations if used for long-term oil production operations at most locations.
The conventional jacket supported or gravity base fixed platform can be designed to be unaffected by high winds and heavy seas. However, it may be economically unwise to place a fixed platform at certain locations, such as those controlled by politically unstable countries. It is also generally not commercially viable to use a fixed platform to conduct production operations from marginal fields. Moreover, conventional fixed platforms designed and constructed to withstand wind and sea forces are generally not made to withstand forces generated by collisions with large icebergs or ice floes. Therefore, massive fixed structures that are many times larger and stronger and many times more expensive than conventional fixed structures would be required for use in iceberg infested waters.
In particular, neither conventional floating nor conventional fixed platforms are well suited for oil or gas production in a location such as the Hibernia oil field off the coast of Newfoundland, Canada, which is subject to relatively severe environmental conditions and periodic intrusion of medium to large size icebergs (height 14-46 meters above water surface, draught of 30-90 meters).
It is considered by the inventors that a production platform operating in a floating mode could not be used efficiently at Hibernia or similar locations, due to the anticipated high occurrence of production stoppages caused by actual or forecast adverse weather and sea conditions. It is also considered that a fixed structure designed to withstand relatively infrequent iceberg collisions would be financially unattractive, due to the very high capital cost of such a structure. Furthermore, it is not clear whether a fixed structure can be built to withstand the forces generated by iceberg collisions.